The terms ‘trunk’ and ‘proboscis’ have been applied to a heterogeneous array of bits danging off vertebrate faces such as the bulbous nose of male Proboscis Monkeys, the pendulous resonating chamber of male Elephant Seals, the hydrostatic facial tentacle of proper elephants, and even the chins of Elephantnose Fish. Milewski & Dierenfeld (2013) argue the terminology has become “dysfunctionally vague” and in need of a massive overhaul. The authors trash the term ‘trunk’ as it is vague and redundant (being a synonym for ‘torso’) and lacks precedence over ‘proboscis’, which was originally applied to elephants and roughly translates as ‘forager’. Due to this etymology and the extreme morphology exhibited by elephants, the term ‘proboscis’ was redefined to be a tubular extension of nasal and lip musculature capable of grasping food (Milewski & Dierenfeld 2013). The only extant non-Proboscideans to fit this definition are tapirs, and while numerous extinct mammals have been interpreted with proboscides (Palorchestes, Cadurcodon, Brachycrus, Macrauchenia, Pyrotherium) only Astrapotherium has a good case for having a proboscis under this new definition, since it doesn’t appear to have been capable of feeding itself otherwise.
As for other nasal structures that are incapable of grasping food, those have now been termed ‘prorhiscis’. These, uh, ‘prorhiscides’ (?) have disparate functions ranging from a directional sense of smell in Elephant-Shrews, dust filtration in Saiga, thermoregulation and water conservation in Dik-Diks, amplification of roars in Elephant Seals, and possibly acting as a buoy for diving Moose (Milewski & Dierenfeld 2012). There are other weird structures formerly labeled ‘proboscis’ in fish, but that’s really a story for a different time.
Gingerich et al. (2005) interpreted the extinct whale Makaracetus as having a “trunk” or “short, muscular proboscis” but confusingly referenced tapirs, manatees and walruses as “imperfect models”. Were they proposing an elephantine proboscis, prehensile lips, or something in between? This is a great example of how the now-archaic terminology was “dysfunctionally vague” and in light of Milewski and Dierenfeld’s efforts, it’s time for a reassessment.
Makaracetus is classified as a protocetid and appears fairly similar to species such as Artiocetus (above), although with four pronounced differences: a nasal vestibule extending to the end of the snout, a downward-deflected rostrum with two rather than three pairs of incisors, “extraordinary” fossae (labeled LFM, LFP above) suggesting massive facial muscles, and enlarged antorbital canals (labeled AF above) indicating increased blood supply to the end of the snout (Gingerich et al. 2005). Clearly something odd was growing on the face of Makaracetus, but without living protocetids, it’s a bit hard to tell just what. I’m not even certain if a Tapir-style proboscis can be distinguished from a prorhiscis without live specimens, so, unfortunately, it appears there’s just no way of knowing things for certain. That won’t stop me from rampantly speculating.
Imperfect models indeed. Gingerich et al. (2005) compared Makaracetus and Tapirs on the basis of expanded nasal vestibules, however, the nasals of Makaracetus are only around half as retracted. Makaracetus and Manatees were compared on the basis of being aquatic and having down-turned rostrums, which Gingerich et al. suggested to be an indicator of benthic feeding. Walruses were suggested as an ecological model, namely as a species that uses facial muscles to prey on bivalves, despite having some very different anatomy. The proposed ecology seems reasonable — although it requires better-known teeth to confirm — but as for what was happening with the nose of Makaracetus, there are some more interesting models.
Superficially, Makaracetus looks kinda camel-y. The rostra of the two species, while differing considerably in depth, appear to be deflected downward at a similar relative place and to a similar degree. Camels have one less pair of incisors than Makaracetus and, oddly, the placements of the remaining incisor pair, canine, and first premolar are comparable. The antorbital foramen (“AF”) of the camel appears to be larger than that of Makaracetus. There are of course several pronounced differences between the two, namely, the fossae in Makaracetus (LFM, LFP) do not appear to have equivalents in camels and, perhaps as a result of these structures, Makaracetus has a rostrum that looks spoon-like when viewed above and camels don’t. Makaracetus and camels undoubtedly had enlarged noses for very different reasons, but this comparison makes a proboscis or even prorhiscis seem unlikely in the whale.
But I’m not quite done with Makaracetus yet, I haven’t even talked about pigs and moose! That will have to wait for a followup to this increasingly out-of-control article.
Gingerich, P. et al. (2005) Makaracetus bidens, a New Protocetid Archaeocete (Mammalia, Cetacea) from the Early Middle Eocene of Balochistan (Pakistan). Contributions from the Museum of Paleontology 31(9) 197—210. Available
Gingerich, P. et al. (2001) Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from Pakistan. Science 293 2239—2242. Available
Husar, S. (1978) Trichechus manatus. Mammalian Species 93 1-5. Available
Milewski, A. & Dierenfeld, E. (2013) A structural and functional comparison of the proboscis between tapirs and other extant and extinct vertebrates. Integrative Zoology doi: 10.1111/j.1749-4877.2012.00315.x
Witmer, L. (1999) The proboscis of tapirs (Mammalia: Perissodactyla): a case study in novel narial anatomy. Journal of Zoology 249 249—267. Available